Kilometer-Scale Ion-Photon Entanglement with a Metastable $^{88}$Sr$^{+}$ Qubit

TL;DR

This paper demonstrates kilometer-scale ion-photon entanglement using a metastable strontium ion, showing high fidelity transmission over urban fiber networks, advancing quantum communication technology.

Contribution

It reports the first kilometer-scale entanglement between a strontium ion and a photon, with high fidelity after fiber transmission, suitable for quantum networking.

Findings

Fidelity of 0.949(4) in lab conditions

Fidelity of 0.929(5) after 2.8 km fiber transmission

Ion-photon entanglement at 1092 nm suitable for telecom infrastructure

Abstract

We demonstrate entanglement between the polarization of an infrared photon and a metastable $^{88}$Sr$^+$ ion qubit. This entanglement persists after transmitting the photon over a $2.8\:$km long commercial fiber deployed in an urban environment. Tomography of the ion-photon entangled state yields a fidelity of $0.949(4)$ within the laboratory and $0.929(5)$ after fiber transmission, not corrected for readout errors. Our results establish the Strontium ion as a promising candidate for metropolitan-scale quantum networking based on an atomic transition at $1092\:$nm, a wavelength compatible with existing telecom fiber infrastructure.